Lithographic apparatus and device manufacturing method

ABSTRACT

In a lithographic projection apparatus, a liquid supply system maintains liquid in a space between a projection system of the lithographic projection apparatus and a substrate. A sensor positioned on a substrate table, which holds the substrate, is configured to be exposed to radiation when immersed in liquid (e.g., under the same conditions as the substrate will be exposed to radiation). By having a surface of an absorption element of the sensor, that is to be in contact with liquid, formed of no more than one metal type, long life of the sensor may be obtained.

This application claims priority from European patent application EP03255228.3, filed Jul. 24, 2003, which is incorporated herein in itsentirety.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective numerical aperture (NA) of thesystem and also increasing the depth of focus.) Other immersion liquidshave been proposed, including water with solid particles (e.g. quartz)suspended therein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example United States patent U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in-and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

A conventional lithographic projection apparatus requires one or moresensors on the substrate table so that, for example, the substrate tablewhich carries the substrate can be correctly positioned relative to theprojection beam. These sensors typically include a Transmission ImageSensor (TIS) which is a sensor that is used to measure the position atsubstrate level of a projected aerial image of a mark pattern at thereticle level (mask). Typically, the projected image at substrate levelis a line pattern with a line width similar to projection beamwavelength. The TIS measures these mask patterns by using a transmissionpattern with a radiation sensor underneath. The sensor data is used tomeasure the position of the mask with respect to the position of thesubstrate table in six degrees of freedom. Also the magnification andscaling of the projected mask pattern are measured, since four points onthe mask are used for the measurement. As the sensor should also becapable of measuring the pattern positions and influences of allillumination settings (sigma, projection system NA, all masks (binary,PSM, . . . )), a small line width is required. Furthermore, the sensoris also used to measure/monitor the optical performance of theapparatus. Different measurements are implemented for measuring pupilshapes, coma, spherical aberration, astigmatism and field curvature. Forthese measurements, different illumination settings are used incombination with different projected images. Also such a sensor may bean Integrated Lens Interferometer At Scanner (ILIAS) which is aninterferometric wavefront measurement system implemented on lithographytools. ILIAS performs (static) measurements on lens aberrations (up toZernicke 36) as are needed for system setup and qualification. ILIAS isan on scanner integrated measurement system used for system setup andcalibration. ILIAS is used for monitoring and recalibration of thescanner on a regular basis depending on the machine needs. Also, such asensor may be a dose (spot) sensor or any other type of sensor that maybe used at substrate level. All of these sensors are used at substratelevel and as such are positioned on the substrate table. In order toavoid the need to perform complex predictions about how the immersionliquid will affect the projection beam, it is desirable to illuminatethe one or more sensors under the same conditions as the substrate is tobe imaged, i.e. with immersion liquid in place between the projectionsystem and the sensor.

Sensors of the type mentioned above used in conventional lithographicprojection apparatus typically have an absorption layer positioned overa grating in front of the actual radiation sensor. The absorbing layeris used to ensure that the sensor is a high contrast sensor so thataccurate readings can be made. The absorbing layer has open and closedareas to get a high signal contrast between the radiation transmittedthrough the open patterns and the closed absorbing area. The photosensor below the absorbing layer is normally much larger than the openpatterns in order to measure the radiation for large angles. The ratioof the open pattern area versus the radiation sensitive area in atypical sensor is roughly (1:5600). So it is often important to absorbas much radiation as possible using an absorbing layer on the closedareas above the radiation sensor. Area patterns with a line width of theorder of 200 nm are used. To implement this, a multi-layer structure isused so that the required resolution can easily be achieved. Typicallythe absorption elements of such sensors are made of a plurality oflayers of different metal types. Chromium is the most widely usedbecause it is common in mask production, and has good absorbingproperties for blocking ultra-violet and deep ultra-violet radiation.Aluminum is also used because it has good etch selectivity with respectto chromium and a good optical density. Other metals, both elemental andalloys, may be suitable. Metals are typically used because of their goodelectrical conductivity and optical reflectivity which is useful forsubstrate table height measurements.

In United States patent U.S. Pat. No. 5,825,043, sensors are arrangedabove the substrate table relying on reflection of radiation off thesurface of the substrate table to avoid making sensors resistant toliquid. However, this may result in a loss in accuracy.

SUMMARY

Accordingly, it would be advantageous, for example, to provide accuratesensors for use in an immersion lithographic projection apparatus.

According to an aspect, there is provided a lithographic projectionapparatus arranged to project, using a projection system, a pattern froma patterning device onto a substrate held by a substrate table,comprising:

-   a liquid supply system configured to provide a liquid to a space    between the projection system and the substrate; and-   a sensor mounted on the substrate table and configured to be exposed    by radiation, the sensor including a surface that is to be in    contact with liquid from the liquid supply system during exposure of    the sensor by radiation, the surface comprising one or fewer metal    types.

In an immersion lithographic apparatus, formation of a galvanic cellbetween two dissimilar metal types when in contact with liquid may beprevented or limited, particularly when the immersion liquid is water orwater based. This may allow a sensor to be positioned on the substratetable in an immersion lithographic apparatus. For example, because ofthe required small size of the features of a sensor, if a galvanic cellis allowed to be established, the effectiveness of that sensor mayquickly decrease as a surface of a metal type dissolves away. Thus,avoiding or limiting the establishment of a galvanic cell, a highcontrast sensor may be achieved which provides a long service lifetimein an immersion environment.

In an embodiment, the surface is formed by a continuous layer of onemetal type. By having a layer of one metal type, the use of two metalsis avoided so that even with minor damage to the continuous layer agalvanic cell will not be established. Areas of different absorptioncharacteristics may be provided by having the layer not be uniform inthickness.

In an embodiment, the sensor may comprise a layer of an isolationmaterial. The isolation material may provide a convenient way to providethat different metal types are electrically insulated from one anotheror isolated from the immersion liquid. In an implementation, the layeris continuous but need not be where the two metal types are insulatedfrom one another or isolated from the immersion liquid.

In an embodiment, the sensor comprises a layer of metal which is not ofuniform thickness. In that embodiment, the surface may comprise areas ofthe layer of isolation material and areas of the layer of metal, whichis of one metal type.

If the layer of metal comprises two layers of metal, each of a differentmetal type, then the layer of isolation material may be sandwichedbetween the two layers, each of a different metal type. Also, where thetwo layers of metal are in contact, the layer of isolation material mayform a surface to be in contact with the liquid. In an embodiment, theisolation material comprises an electrical insulation material.

In an embodiment, the surface is formed of a layer of one metal type andthe sensor comprises a further layer of a material other than metal. Thematerial other than metal may be a ceramic.

In an embodiment, the sensor may comprise an absorption element withareas of different absorption characteristics and the surface is formedby the absorption element.

According to a further aspect, there is provided a device manufacturingmethod comprising:

-   projecting a patterned beam of radiation onto a substrate through a    liquid in a space between a projection system of a lithographic    apparatus and a substrate; and-   illuminating a sensor on a substrate table after providing a liquid    between the projection system and the sensor, wherein the sensor    includes a surface in contact with the liquid being formed of one or    fewer metal types.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system in a lithographic projectionapparatus;

FIG. 4 depicts another liquid supply system in a lithographic projectionapparatus;

FIG. 5 illustrates another liquid supply system as well as a sensoraccording to an embodiment of the present invention;

FIG. 6 illustrates a sensor according to the first embodiment of thepresent invention;

FIG. 7 illustrates a sensor according to a second embodiment of thepresent invention;

FIG. 8 illustrates a sensor according to a third embodiment of thepresent invention;

FIG. 9 illustrates a sensor according to a fourth embodiment of thepresent invention; and

FIG. 9A illustrates a further sensor according to a fourth embodiment ofthe present invention.

DETAILED DESCRIPTION

Embodiment 1

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   an illumination system (illuminator) IL configured to condition a    radiation beam PB (e.g. UV radiation or DUV radiation).-   a support structure (e.g. a mask table) MT constructed to support a    patterning device (e.g. a mask) MA and connected to a first    positioner PM configured to accurately position the patterning    device in accordance with certain parameters;-   a substrate table (e.g. a wafer table) WT constructed to hold a    substrate (e.g. a resist-coated wafer) W and connected to a second    positioner PW configured to accurately position the substrate in    accordance with certain parameters; and-   a projection system (e.g. a refractive projection lens system) PL    configured to project a pattern imparted to the radiation beam PB by    patterning device MA onto a target portion C (e.g. comprising one or    more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system.”

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AM for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam PB passes through the projection systemPL, which focuses the beam onto a target portion C of the substrate W.With the aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam PB.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamPB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2. Although the substrate alignment marksas illustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the mask MA, the mask alignment marks may be located between thedies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 5 shows a liquid reservoir 10 between the projection system PL anda sensor 20 which is positioned on the substrate stage WT. The liquidreservoir 10 is filled with a liquid 11 having a relatively highrefractive index, e.g. water, provided via inlet/outlet ducts 13. Theliquid has the effect that the radiation of the projection beam is ashorter wavelength in the liquid than in air or in a vacuum, allowingsmaller features to be resolved. It is well known that the resolutionlimit of a projection system is determined, inter alia, by thewavelength of the projection beam and the numerical aperture of thesystem. The presence of the liquid may also be regarded as increasingthe effective numerical aperture. Furthermore, at fixed numericalaperture, the liquid is effective to increase the depth of field.

In an embodiment, the reservoir 10 forms a contactless seal to thesubstrate around the image field of the projection system PL so that theliquid is confined to fill the space between the substrate's primarysurface, which faces the projection system PL, and the final opticalelement of the projection system PL. The reservoir is formed by a sealmember 12 positioned below and surrounding the final element of theprojection system PL. Thus, the liquid supply system provides liquid ononly a localized area of the substrate. The seal member 12 forms part ofthe liquid supply system for filling the space between the final elementof the projection system and the sensor 20 (or substrate) with a liquid.This liquid is brought into the space below the projection system andwithin the seal member 12. The seal member 12 extends a little above thebottom element of the projection system and the liquid rises above thefinal element so that a buffer of liquid is provided. The seal member 12has an inner periphery that at the upper end closely conforms to theshape of the projection system or the final elements thereof and may,e.g. be round. At the bottom the inner periphery forms an aperture whichclosely conforms to the shape of the image field, e.g. rectangular,though this is not necessarily so. The projection beam passes throughthis aperture.

The liquid 11 is confined in the reservoir 10 by a seal device 16. Asillustrated in FIG. 2, the seal device is a contactless seal, i.e. a gasseal. The gas seal is formed by gas, e.g. air or synthetic air, providedunder pressure via inlet 15 to the gap between seal member 12 andsubstrate W and extracted by first outlet 14. The over pressure on thegas inlet 15, vacuum level on the first outlet 14 and the geometry ofthe gap are arranged so that there is a high-velocity gas flow inwardstowards the optical axis of the apparatus that confines the liquid 11.As with any seal, some liquid is likely to escape, for example up thefirst outlet 14.

FIGS. 2 to 4 also depict a liquid reservoir defined by inlet(s) IN,outlet(s) OUT, the substrate W and the final element of projectionsystem PL. Like the liquid supply system of FIG. 5 the liquid supplysystems illustrated in FIGS. 2 to 4, comprising inlet(s) IN andoutlet(s) OUT, supplies liquid to a space between the final element ofthe projection system and a localized area of the primary surface of thesubstrate.

All of the liquid supply systems of FIGS. 2 to 4 and FIG. 5 can be usedwith the sensor 20 which is illustrated in more detail in FIG. 6. InFIG. 6 the liquid supply system is not illustrated but the final elementof the projection system 30 is depicted as is the liquid reservoir,which is filled with immersion liquid 11.

Referring to FIG. 6, the sensor 20 comprises a sensor element detector40, a transmissive sensor grating 45 and an absorption element 100. Theabsorption element 100 is used to enhance the sensor contrast and thusthe overall sensor performance. The contrast in the sensor due to theabsorption element 100 is the ratio of the radiation transmitted throughan open area of the pattern in relation to the amount of radiation thatis transmitted through the closed area i.e. the areas which are covered.The behavior of the absorption element is mainly driven by the ratio ofthe open area of the transmissive sensor grating 45 and the area of theabsorption element 100 above the sensor element detector 40. For a TIS,the ratio of the open grating area to closed absorbing area is 1:5600.In order to obtain a 0.1% contribution of the radiation transmittedthrough the absorption element 100 an optical density of 0.001/5600˜2e-7is needed.

The transmissive sensor grating 45 is used for convolution of theprojected aerial image of a corresponding pattern at reticle level (4 or5 times larger than the pattern on the sensor). The convolution of thetransmissive sensor grating 45 with the projected aerial image of thepattern at reticle level provides an intensity profile depending on theposition of the transmission sensor grating 45 at substrate level. Withthe intensity data at different substrate table positions the positionand shape of the aerial image can be calculated.

The sensor element detector 40 transforms the radiation that istransmitted through the open area of the grating into an electricalsignal. The total amount of radiation that is transmitted through thegrating 45 and the absorption element 100 contributes to the totalelectrical signal. In order to get a good sensor performance, the amountof radiation that transmits through the absorption element 100 should beminimized (less than 0.1% of the radiation transmitted through the openarea). If the optical density of the absorption element 100 is toolarge, the sensor performance will likely degrade. The purpose of theabsorption element 100 is thus to absorb part of the energy of theprojection beam PB so that the sensor can achieve sufficient contrast byproviding areas of different absorption characteristics.

In an embodiment, the absorption element 100 is made of at least onemetal layer (shown in FIG. 6 as two layers 105, 107) such as aluminumand/or chromium (or alloys thereof) but may be made of one or morelayers of any metal(s). Al and Cr are particularly efficient atabsorbing projection beam radiation and may be advantageousmanufacturing and processing resources.

The total thickness of the metal layer(s) is of the order of 200 nm. Inan Al/Cr multi-layer, the Al layer should typically be 100 nm and the Crlayer 90 nm thick. The width of the areas not covered is of the order of100 to 300 nm for TIS and 2–5 μm for ELIAS. A problem with using twolayers, each of different metal types in the absorption element, is thata galvanic cell may be set up when the two metal types are in electricalcontact and in contact with an electrolyte, i.e., the immersion liquid.In such a galvanic cell the less noble metal (aluminium) reacts as inreaction 1:Al→Al³⁺+3e⁻  (reaction 1)

The three electrons generated in reaction 1 move to thechromium-immersion liquid interface where reaction 2 takes place. In thecase of an immersion liquid based on water, the reaction is:2H₂O+O₂+4e⁻→4OH⁻  (reaction 2)For immersion liquids other than water, a slightly different reactiontakes place. When the foregoing reactions are summed, the followingreaction takes place:4Al+6H₂O+3O₂→4Al³⁺+12OH⁻  (reaction 3)

As a rule of thumb, the rate of the overall reaction is influenced,among others, by the metals involved. The closer their positions in thegalvanic series, the lower the corrosion rate will be. Metals close toaluminum in the galvanic series, such as zinc or cadmium, typically havedrawbacks that may make them not suitable for use in this application toreplace the chromium.

In an embodiment, the establishment of such a galvanic cell is preventedor limited by the use of a continuous layer 120 of an isolation materialsuch as SiO₂ or SiN typically between 20 and 100 nm in thickness. Thelayer 120 need not be continuous over areas where only layer 105 wouldbe in contact with the immersion liquid; it need only be continuouswhere without it, layers 105 and 107 would together come into contactwith the immersion liquid. In an implementation, a continuous layerusing all of layer 105 is used as it prevents problems with thecompatibility of layer 105 with the immersion liquid due to pinholes andscratches in layer 105. The isolation material may either be liquidimpermeable (either electrically insulative or conductive) or anelectrical insulator and must be sufficiently transparent to theradiation with which the sensor is irradiated. The layer 120 covers bothmetal layers 105, 107 of the absorption element 100 (i.e. an outersurface of the absorption element 100, which is in contact with theimmersion liquid during exposure of the sensor, is formed by thecontinuous layer) such that a galvanic reaction between the two metallayers or even between two different metal types in a single layercannot proceed because the second metal layer 107 (the chromium layer)or the two different metal types in a single layer is isolated from theimmersion liquid 11 so that reaction 2 cannot proceed.

According to an embodiment, the formation of a galvanic cell between twodissimilar metal types when immersed in immersion liquid can beprevented, particularly when the immersion liquid is water or waterbased. This allows the sensor to be positioned on the substrate table.Because of the required small size of the features on the absorptionelement, if a galvanic cell is allowed to be established, theeffectiveness of that absorption element will quickly decrease as one ofthe metal types dissolves away. Thus, a high contrast sensor may beachieved which provides a long service lifetime in an immersionenvironment.

As used herein, the term metal type means an elemental metal or analloy. The isolation material provides a convenient way to ensure thatdifferent metal types can be electrically insulated from one another orisolated from the immersion liquid.

As will be appreciated, each of the two metal layers 105, 107 may bemade of different types of metal and indeed this embodiment can also beused with only a single layer of metal which may be made of one or moretypes of metal.

Embodiment 2

A second embodiment will now be described with reference to FIG. 7 andis the same as the first embodiment except as described below.

In the second embodiment, the absorption element 100 comprises two metallayers 105, 107. However, in the second embodiment, the isolation layer120 is made of an electrical insulation material and is sandwichedbetween the first and second layers 105, 107. Again, the isolation layeris only necessary where the stack of layers 105 and 107 would, withoutit, come into contact with the immersion liquid but, in animplementation, a continuous isolation layer is used to deal withimperfections in layer 105. Thus, the surface of the absorption element100 is formed partly of the first metal layer 105 and partly of thelayer of insulation material 120. In this embodiment the function of thelayer of insulation material 120 is to electrically isolate the firstand second metal layers 105, 107 from each other such that no galvaniccouple exists. In this embodiment, the first metal layer 105 should bemade of a single metal type i.e. an elemental metal or an alloy but notof two elemental metals or two alloys which are not fully alloyed. Thesecond metal layer 107 may be made of one or more metal types.

Embodiment 3

A third embodiment will now be described with reference to FIG. 8 and isthe same as the first embodiment save as described below.

In the third embodiment, there is no layer of insulation material 120.There are two layers 105, 108, one of which is a metal layer 105, forexample Cr, and another of which is a non-metal layer 108, such as aceramic, for example a metal carbide or nitride such as TiN. The layers105, 108 may be either way around. This combination also has goodresistance to galvanic corrosion and has an advantage of being easy tomanufacture as each layer needs only to be about 100 nm thick.

Embodiment 4

A fourth embodiment will now be described with reference to FIGS. 9 and9A and is the same as the first embodiment except as described below.

In the fourth embodiment, the formation of a galvanic couple isprevented or limited by the use of only one metal type in the absorptionelement 100. The single metal type may be deposited in a layer 105. Theareas of different absorption characteristics are formed by depositingthe metal layer 105 in different thicknesses (as shown in FIG. 9A as acontinuous layer), which includes areas where the layer is not depositedat all (as shown in FIG. 9), so that the thickness of the layer 105 isnot uniform. As only one metal type is exposed to the immersion liquid,no galvanic couple exists. Thus, the use of two metals in the absorptionelement is completely avoided so that even with minor damage to thecontinuous layer a galvanic cell will not be established.

In the above described embodiments, the metal layers 105, 107 may be ofvarious thicknesses over the area of the sensor. The variationthicknesses include areas where the layer is not deposited at all (as inFIGS. 6, 7 and 8). The first and second embodiments have been describedwith the metal layers either in existence in a given area at a certainthickness, or not existent in an area at all; this need not be the caseand the thicknesses of the layers may vary over the area of the sensor.

In an embodiment, the transmissive sensor grating 45 and the substratetable WT are made of non conductive materials. This has an advantageousas a galvanic couple cannot then be set up between the metal layers 105,107 deposited on the substrate table WT or transmissive sensor grating45 and other metals which are also in contact with the immersion liquid.

The above described methods of avoiding the formation of galvanic cellscan be applied to any parts of any sensors or other devices which areimmersed in immersion liquid.

Another liquid supply system which has been proposed is to provide theliquid supply system with a seal member which extends along at least apart of a boundary of the space between the final element of theprojection system and the substrate table. The seal member issubstantially stationary relative to the projection system in the XYplane though there may be some relative movement in the Z direction (inthe direction of the optical axis). A seal is formed between the sealmember and the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal. Such a system with a gas seal isdisclosed in U.S. patent application U.S. Ser. No. 10/705,783, herebyincorporated in its entirety by reference.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5–20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, an embodiment of the invention may takethe form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. The descriptionsabove are intended to be illustrative, not limiting. Thus, it will beapparent to one skilled in the art that modifications may be made to theinvention as described without departing from the scope of the claimsset out below.

1. A lithographic projection apparatus arranged to project, using aprojection system, a pattern from a patterning device onto a substrateheld by a substrate table, comprising: a liquid supply system configuredto provide a liquid to a space between the projection system and thesubstrate; and a sensor mounted on the substrate table and configured tobe exposed by and sense ultraviolet radiation, the sensor including asurface that is to be in contact with liquid from the liquid supplysystem during exposure of the sensor by radiation, the surfacecomprising no more than one metal type.
 2. The lithographic projectionapparatus according to claim 1, wherein the surface is formed by acontinuous layer of one metal type.
 3. The lithographic projectionapparatus according to claim 2, wherein the continuous layer is notuniform in thickness to provide areas with different absorptioncharacteristics.
 4. The lithographic projection apparatus according toclaim 1, wherein the surface comprises a layer of one metal type and thesensor comprises a further layer of a material other than metal.
 5. Thelithographic projection apparatus according to claim 4, wherein thematerial other than metal comprises a ceramic.
 6. The lithographicprojection apparatus according to claim 1, wherein the sensor comprisesa layer of an isolation material.
 7. The lithographic projectionapparatus according to claim 6, wherein the sensor comprises a layer ofmetal which is not of uniform thickness.
 8. The lithographic projectionapparatus according to claim 7, wherein the surface comprises areas ofthe layer of isolation material and areas of the layer of metal, whichis of one metal type.
 9. The lithographic projection apparatus accordingto claim 7, wherein the layer of metal comprises two layers of metal,each of a different metal type.
 10. The lithographic projectionapparatus according to claim 9, wherein the two layers of metal are incontact and the layer of isolation material forms a surface to be incontact with the liquid.
 11. The lithographic projection apparatusaccording to claim 10, wherein the isolation material comprises anelectrical insulation material.
 12. The lithographic projectionapparatus according to claim 10, wherein the layer of isolation materialblocks the two layers of metal from contact with the liquid.
 13. Thelithographic projection apparatus according to claim 10, wherein thelayer of isolation material blocks one of the layers of metal fromcontact with the liquid.
 14. The lithographic projection apparatusaccording to claim 9, wherein the layer of isolation material issandwiched between the two layers of metal.
 15. The lithographicprojection apparatus according to claim 14, wherein the isolationmaterial comprises an electrical insulation material.
 16. Thelithographic projection apparatus according to claim 1, wherein thesensor comprises an absorption element with areas of differentabsorption characteristics and the surface is formed by the absorptionelement.
 17. A device manufacturing method, comprising: projecting apatterned beam of radiation onto a substrate through a liquid in a spacebetween a projection system of a lithographic apparatus and a substrate;and illuminating a sensor on a substrate table after providing a liquidbetween the projection system and the sensor, wherein the sensor isconfigured to sense ultraviolet radiation and includes a surface incontact with the liquid being formed of no more than one metal type. 18.The method according to claim 17, wherein the sensor comprises a layerof an isolation material.
 19. The method according to claim 18, whereinthe sensor comprises a layer of metal comprising two layers of metal,each of a different metal type.
 20. The method according to claim 19,wherein the two layers of metal are in contact and the layer ofisolation material forms a surface to be in contact with the liquid. 21.The method according to claim 20, wherein the layer of isolationmaterial blocks the two layers of metal from contact with the liquid.22. The method according to claim 19, wherein the layer of isolationmaterial is sandwiched between the two layers of metal.
 23. The methodaccording to claim 22, wherein the isolation material comprises anelectrical insulation material.
 24. The method according to claim 22,wherein the layer of isolation material blocks one of the layers ofmetal from contact with the liquid.
 25. The method according to claim17, wherein the sensor comprises a layer of metal which is not ofuniform thickness.